Main storage or RAM. Dates from the days of ferrite-core memory;
now archaic as techspeak most places outside IBM, but also still used in
the Unix community and by old-time hackers or those who would sound like
them. Some derived idioms are quite current; in
core, for example, means ‘in memory’ (as opposed to
‘on disk’), and both core dump and the
core image or core file produced by one are terms in favor.
Some varieties of Commonwealth hackish prefer
store.